The present invention relates to vehicle control modules, and more particularly to a vehicle control module that controls deployment of squib-activated vehicle devices.
Control modules for controlling deployment of vehicle devices, such as airbags, in vehicles rely on application specific integrated circuits (ASICs) populated with one or more diagnostic loops. In these systems, diagnostic loops associated with vehicle devices each contain a squib, which has a very low resistance on the order of 2 ohms, that explodes or fires when a high current is sent through it in response to a detected vehicle condition. In airbag systems, firing the squib causes the airbag associated with that squib to inflate. Squibs can be associated with other vehicle safety devices as well, but airbags will be discussed below for simplicity.
Many control modules are designed so that they can contain a high and low number of loops containing squibs, thereby allowing the same control module to be used in different vehicle designs. For example, a given control module may contain either thirteen squib loops (high) or seven squib loops (low). If the module contains seven squib loops, replacement resistors can be placed in all six unused squib loops in place if necessary. These replacement resistors have higher resistance values than the squibs so that diagnostic circuitry within the airbag system can distinguish between the squib and the replacement resistor.
The replacement resistors are used to prevent misbuilds at the vehicle plant and during the service life of the vehicle. For example, to protect the correct configuration of the vehicle, replacement resistors can be used in the seats and headliners in the vehicle. This is done for two reasons. First, seats and headliners containing airbags are designated with a label marked xe2x80x9cAirbag.xe2x80x9d A misbuild may cause the owner to incorrectly believe that a seat or headliner has airbag protection when it actually does not, causing customer dissatisfaction. Second, it is expensive to give away airbags in seats and headliners in a vehicle that is not configured to have such devices. The ASICs prevent these types of misbuilds by detecting the different resistance levels of the squibs and the replacement resistors and making sure that the number of detected squibs and replacement resistors in the control module match the vehicle design.
Previously known designs, which contain a smaller total number of loops, could be diagnosed by one ASIC. Designs having a larger number of loops and having a large difference between the high and low numbers of loops may require more than one ASIC to diagnose. In this case, however, one of the ASICs may be populated solely to detect the replacement resistors. The additional ASIC undesirably increases the cost of the diagnostic circuitry.
There is a desire for a control module that can diagnose the presence of replacement resistors without requiring additional ASICs to conduct the diagnosis.
The present invention is directed to a control module having a diagnostic circuit. The diagnostic circuit contains one or more ASICs having a plurality of loops. The loops may contain either a squib, forming a squib loop, or a replacement resistor, forming a replacement resistor loop. Rather than populating an additional ASIC simply to diagnose the replacement resistor loops, the invention connects a multiplexer to one loop of the ASIC and uses the multiplexer to diagnose the replacement resistor loops.
By using a multiplexer to detect replacement resistors, the invention eliminates the need to include an additional ASIC to diagnose replacement resistor loops. Instead, a single ASIC loop can be used to diagnose multiple replacement resistors.